Communications networks often incorporate maintenance equipment which enable the sectionalization and isolation of faults. The term "fault" is a generic term which denotes an improper operation of a communications network. Sectionalization of a fault is the determination that a fault is originating within a particular section or link in a series of links in the communications network. Such a link may contain a number of different circuits. Isolation of a fault is the determination that a particular piece of communications equipment in the particular link is operating improperly.
To provide fault sectionalization or isolation, predetermined test signals are typically inserted into the circuit under test via a test system. In digital communications systems, the prescribed test and response signals are
generically known as control bits and are transmitted in prescribed time slots. In any event, during the transmission of test and response signals, the communications system is not available for the transmission of data by a customer.
In one well-known fault detection technique, test signals are transmitted from a test location to maintenance equipment situated at other locations in the circuit under test. Upon receipt of these test signals, the maintenance equipment transmits predetermined response signals back to the test location. The receipt of such response signals at the test location then provides an indication that the section of the communications network, including one or more links, is functioning properly. Conversely, improper operation is indicated by the absence of these response signals at the test location. In other applications, the response of the maintenance equipment to the test signals is to provide a "loop-back" whereby the transmit and receive signal paths are connected together. As a result, the test signal transmitted is coupled to the test location. In still other applications, the maintenance equipment could provide an open circuit in the communications network. With the burgeoning growth of communications systems both in number and complexity, the above described test and maintenance procedures significantly reduce the time and expense associated with network maintenance and fault sectionalization and/or isolation.
While the above-described maintenance and test procedures provide a significant reduction in both the time and expense associated with network maintenance and fault correction, the reservation of particular time periods for the transmission of test and response signals, lengthens the transmission time in analog communications systems or reduces the bandwidth available for the user of a digital communications system. These effects are troublesome in particular applications where faster communication or greater bandwidths are desired. While these effects could be eliminated by removing the test and maintenance equipment, such action is undesirable as the time and expense required to restore network outages would be considerably increased. Similarly, the transmission of unrestricted system user data in the time periods reserved for test and maintenance can result inadvertent actuation of the remote maintenance equipment when such data mimics the prescribed test signals. Such actuation would subsequently disrupt the transmission of user data. Accordingly, a technique for precluding the inadvertent actuation of maintenance and test equipment in a communications system would be high desirable.